This invention relates to a process for the production of the polyester poly(trimethylene terephthalate) (PTT) from 1,3-propanediol (PDO) and an aromatic diacid or dialkyl ester thereof by melt polymerization followed by solid state polymerization (SSP). More particularly, this invention relates to an improved method of preparing the melt polymerized prepolymer for solid state polymerization.
Poly(trimethylene terephthalate) is a new polyester with a unique combination of properties that are particularly suitable for carpet and textile fiber applications. The molecular weight of poly(trimethylene terephthalate) required for fiber applications is between 18,000 and 22,000 which is equivalent to an intrinsic viscosity (IV) between 0.80 and 0.94 dl/g as measured in a 60/40 phenol/tetrachloroethane solvent at 30xc2x0 C. Up to the present time, it has been found to be very difficult and very expensive to produce PTT with such an IV and good color by melt phase polymerization alone. Thus, it is desirable to use a combined melt/solid state polymerization process to produce high quality PTT for fiber applications.
First a PTT prepolymer with an intermediate IV is produced by melt polymerization. The prepolymer pellets thus produced are further polymerized in solid state to the desired IV. In the case of the much more well-known poly(ethylene terephthalate) (PET), a resin with a molecular weight of 20,000 (or an IV of 0.63 dl/g) for textile fiber applications can be produced by melt polymerization alone. However, solid state polymerization has been widely used to produce PET resins with IV""s higher than 0.70 dl/g for bottle, food tray, and tire cord applications.
U.S. Pat. Nos. 4,374,975 and 5,408,035 describe a standard continuous SSP process for the production of polyethylene terephthalate (PET) after melt polycondensation. This is essentially the same process which is currently used commercially for PTT. Prepolymer granules or pellets are first crystallized in a crystallization unit. Then the crystallized pellets are dried and annealed in a dryer/annealer apparatus. The dried and annealed pellets are then preheated to the reaction temperature for SSP in the preheater apparatus. Then the pellets are charged to the reactor where they undergo further polycondensation in the solid state as they move down the .reactor and the final solid stated product pellets are discharged from the reactor and then cooled in a product cooler to a temperature suitable for shipping or storage.
PET prepolymer in granular form is first crystallized in the crystallizer at a temperature of 150 to 180xc2x0 C. to raise its sticking temperature. During a residence time of 5 to 30 minutes, depending on the type of the crystallizer used, the crystallinity of the PET prepolymer is increased to 30 to 40 percent. The crystallized PET pellets are then dried and annealed for 1 to 4 hours (typically 2 to 3 hours) in a dryer/annealer. The dryer/annealer is a moving-bed hopper, wherein the prepolymer pellets move down slowly by gravitational force in contact with a stream of hot air or nitrogen which flows upwardly to sweep away the moisture given off by the prepolymer pellets. The moisture content of the prepolymer pellets is reduced to below 0.01 percent in the dryer/annealer with little change in temperature. In some variations of SSP processes (e.g., SSP processes provided by Hosokawa Bepex Corporation of Minneapolis, Minn. and Sinco Engineering S.p.A. of Tortona, Italy), the moving-bed hopper is replaced by a horizontal agitated heater, wherein the prepolymer pellets are slowly propelled forward by the agitators and heated to a substantially higher temperature. The drying is required to prevent hydrolytic degradation during the subsequent preheating and reaction steps, wherein the prepolymer is exposed to SSP reaction temperature. For simplicity, SSP reaction temperature is defined as a temperature equal to or higher than the nominal SSP reactor temperature. Since the preheater must heat the prepolymer to the reaction temperature, the prepolymer is first exposed to the reaction temperature in the preheater. Hydrolytic degradation of a polyester is a depolymerization reaction which lowers the intrinsic viscosity (IV) of the polyester. Although hydrolytic degradation usually does not significantly affect the final product quality, it can substantially increase the SSP time required to achieve the necessary product IV. It should be noted that at temperatures where hydrolytic degradation takes place, polycondensation also takes place. The combined effects may result in a net IV drop or increase. According to U.S. Pat. No. 4,374,975, it is necessary to reduce the moisture content of the prepolymer to below 0.01 percent prior to its exposure to the reaction temperature to limit the hydrolytic degradation of the PET granules to an acceptable extent. During drying, annealing (the morphological transformation which further increases crystallinity and reduces pellet sticking tendency) also takes place.
The dried and annealed PET granules are then preheated to the SSP reaction temperature, usually between 210 and 220xc2x0 C. During the preheating step, the PET granules become sticky because of the rapid rise of polymer temperature. Therefore a preheater, which can be a fluid bed or an agitated heat transfer unit, must also provide agitation or forced motion to prevent agglomeration of PET granules. The residence time in the preheater ranges from a few minutes to about 60 minutes, depending on the type of the preheater used.
The preheated pellets discharged from the preheater enter the moving-bed solid state reactor. Inside the reactor, the pellets move downward slowly by gravitational force while undergoing polycondensation in the solid state. The reactor provides sufficient residence time for the polyester to achieve the required product IV.
Thus it can be seen that the standard or conventional SSP process for PET comprises several prereaction steps with a long total treatment time (up to several hours) and is characterized by the minimal moisture content (below 0.01 percent) of the prepolymer when the prepolymer is exposed to the reaction temperature. The purpose of these prereaction steps is to reduce polymer sticking tendency and to prevent hydrolytic degradation. If amorphous PET prepolymer is directly exposed to the preheating or reaction temperature of 200 to 230xc2x0, severe pellet sticking or lumping and hydrolytic degradation will occur. The importance of drying the PET before exposing it to the reaction temperature is demonstrated in U.S. Pat. No. 4,374,975. The examples show that an insufficiently predried resin with a moisture content of higher than 0.01 percent at the time it is exposed to the reaction temperature underwent significant IV drop within the first 30 minutes of the exposure.
Surprisingly, I have discovered, in contrast to the behavior of PET in the SSP process, PTT prepolymer pellets can be directly exposed to the preheating or reaction temperature without significant polymer sticking and depolymerization problems. This makes it possible to drastically simplify the prereaction steps for the SSP of PTT. The present invention describes such a simplified SSP process.
The present invention is an improvement upon the process for producing poly(trimethylene terephthalate) (PTT) wherein 1,3-propanediol (PDO), and optionally other diols, and an aromatic diacid or diester thereof, and optionally other diacids or diesters, are esterified or transesterified and the esterification or transesterification product is polycondensed to produce a prepolymer which is then solid state polymerized to produce a polymer with a desired intrinsic viscosity (IV). The prior art process includes the SSP prereaction steps of crystallization, drying/annealing, and preheating. The improvement in the present invention comprises crystallizing and preheating the PTT polymer in one step, without drying/annealing of the polymer. Thus, the temperature of the prepolymer is exposed to the SSP reaction temperature but the prepolymer still has a substantial moisture content, i.e., greater than 0.01% by weight, when it is first exposed to the reaction temperature in the crystallization/preheating vessel.